Lab 3

Lab 3: Audio Amplifier

Introduction

For reference, here is the lab manual that was followed. Circuit diagrams given in the manual will not be reproduced here. This lab goes over various types of audio amplifiers. There are five designs that were tested and all five have their own section below.

Metrics for amplifiers:

We won't be measuring the efficiency in this lab as its difficult/tedious to do.

But why do we even need another amp? Didn't we explore the "all powerful CE amplifier" last lab? Basically most amplifier types cannot push a lot of current even if they can generate large voltage swings. And the CE amp studied in lab 2 is hopeless with low resistance loads, decreasing its gain drastically. At 1kHz the gain goes to 1.84 V/V for a 10 Ohm resistor, not nearly enough to amplify the miniscule signals that will be coming out of a radio.

The function generator suffers from the same problem. Applying a 2.2V signal to the speaker causes the actual voltage to drop to 640 mV. This is due to the generator normalizing its voltages for a 50 ohm load. And so while the CE amp may be all powerful, everyone needs a friend every once in a while. This brings us to our first amp.

Common-Emitter Amplifier into Common-Collector Amplifier

To fix this problem with the CE amp, we can follow it by a CC amp as can be seen below. This remedies the load resistance problem as the CC amp has extremely high input impedance. It's primary problem is a gain near or below unity, but this is fixed by the preceding CE.

First thing first, testing each amp separately. The CE amp was tested rather extensively in lab 2, its waveforms are clean with minimal distortion. This holds true even at relatively high input amplitudes as can be seen in the next image. A 2 Vpp input amplitude comes out as a fairly clean 4 Vpp output which would be usable but isn't much of an amplifier

Above I mentioned the CC amp, but switching to this does barely any better. 800 mVpp in with 4 Vpp out is double the performance but still not useful yet.

Setting up the CC-CE amp combo takes this to 45 mVpp in and 1.8 Vpp out, a huge improvement. A 31.5 V/V gain is a pretty huge improvement over 2 and 4.25. The waveform has mild distortion which can be seen below but it couldn't be heard and it's possible it is just instrumentation error.

Turning off the function generator and measuring the current consumed by checking voltage across a dummy resistor shows that this amp draw 0.88 W. This quiescent draw is extremely high considering the load is at most half a watt, hence the need for alternative amps.

The class B push-pull amplifier

This type of amp earns it's name because two transistors are used in the design, one that pushes current and the other that sinks it. This is reminiscent of CMOS construction except the magnitude of the current can be controlled in this case because the transistors are BJTs instead of mosfets. See the image below as an example, shamelessly ripped from here. Note: this isn't representative of what was actually built and tested.

As promised, this configuration fixes the quiescent power draw of the CE-CC amp, using only 36.16 mW. Everything comes at a cost, as this amp as some serious distortion due to the turn-on voltage of the transistors. This can be seen below.

Fear not theres a solution to the distortion.

The class AB push-pull amplifier

Its like the B push-pull's younger brother. Smarter, cooler, and gets all the babes. This amp uses additional diodes to handle the turn-on voltage. A downside of this is a slight increase in the quiescent draw, from 36.16 mW up to 41.78 mW but thats a small price to pay for pretty squiggles. See below for the output waveform, mostly undistorted.

The gain of this system isn't great, about 0.7 V/V, but it can serve as a worthy replacement for the power hungry CC amp. Preamping the signal with a CE brings the total gain to around 15 V/V with 42.4 mVpp in and 600 mVpp out.

The powerhouse: LM386

With only three external components, the LM386 achieves a gain of 20.5 V/V, 4.2 Vpp out with 206.8 mVpp in. The circuit was designed for 20 V/V so the result of 20.5 V/V is quite accurate. I failed to probe this one, but you can rest assured TI didn't put out an amp with serious distortion. Checking the quiescent power, the LM386 draws around 45mW. The internals are an AB push-pull so this draw being almost identical to our AB push-pull makes sense.

The jack of all trades: OP-Amp

Setting up the OP-amp directly from the lab manual yields a gain of 80.4 V/V. Thats delivered to a 1k ohm load, which no speaker ever is. OP-Amps have the distinct problem of not being able to pump much current so the op-amp needs to be followed by the AB push-pull from above. This new two stage amp has a total gain of around 45.13 V/V into the 8 ohm load which is great but sadly the quiescent draw goes up to 401.7 mW.

Conclusions and Reflections

Five different amplifier stages were tested here all of which have their merits.

CE:

This makes the CE a great choice as a pre-amp for the final stage, but it has no hope of being a solo amplifier

OP-Amp:

This makes the OP-AMP a great choice as a pre-amp for the final stage, but it has no hope of being a solo amplifier. Specifically its good as it is trivial to tune the gain.

CC:

This makes the CC a decent choice for the final stage, but it also has no hope of being a solo amplifier

Class B Push Pull:

This makes the Class B a semi-decent choice for the final stage, but it also has no hope of being a solo amplifier

Class AB Push Pull:

This makes the Class AB the absolute best choice for the final stage, but it also has no hope of being a solo amplifier

LM386:

This makes the LM386 a great choice for the whole audio amp, one of the best in every way.

So for pre-amps we have CE and the op-amp, while for buffer stages we have CC, B, and AB. The lm386 sits in a class of its own as it serves as both a pre-amp and a buffer amp. Personally I'll be using it almost exclusively due to its low external part count and high gain.